Dual drill rod drilling systems (“dual rod”) for use in directional drilling having an inner rod and an outer rod are known. A typical dual rod drilling system is generally configured to drive into the ground a series of drill rods joined end-to-end to form a drill string. At the end of the drill string is a rotating drilling tool or drill bit. A dual rod drilling system typically includes a first drive mechanism that controls rotation of a drill bit and a second drive mechanism that controls rotation of a steering element. When a straight hole is drilled with a dual rod drilling system, the first and second drive mechanisms are concurrently operated such that both the drill bit and the steering element are rotated as the drill string is thrust into the ground. When a directional change is needed, because the steering element is axially misaligned with the drill string, the drive mechanism that controls the steering element is stopped and the drill string is thrust further into the ground while the drive mechanism that controls the drill bit is rotated. This causes the drill bit to deviate from a straight path and follow the direction dictated by the steering element.
Dual rod drilling systems also use drilling fluid that is passed internally within the drill rods for cooling of the drill bit and also for transporting cuttings within the drill hole. Therefore, to ensure proper operation, it is important to reduce obstructions within the drilling fluid flow path. However, this can be difficult due to the unavoidable relative longitudinal offsets between inner and outer drill rods within the drill string.
Further, the inner and outer drill rods of each drill rod assembly can have variations in length resulting from manufacturing tolerances. Because of the length variations, drill rod assemblies are designed such that the overall length of interconnected inner drill rods are never longer than the overall length of interconnected outer drill rods. If the interconnected inner drill rods were longer than the outer drill rods, the inner rods would collide while the outer drill rods were being coupled together, causing damage to one or both of the inner and outer drill rods. Accordingly, by design, the length of interconnected inner drill rods is slightly less than the length of interconnected outer drill rods. However, this design requirement results in a situation where certain portions of the drill string, e.g., the inner drill rods, contact the outer drill rods and obstruct the fluid flow path. This results in being able to send less drilling fluid to the drill head and/or possible damage to portions of the drill string. Therefore, improvements in maintaining an open drilling fluid flow path are needed.
To drive the drill bit with the first drive mechanism, flexible and/or bent drive shafts have been used in order to allow steering and still facilitate torque transfer. Other designs have used a coupling (sometimes referred to as a “transmission”) so as to allow misalignment between a straight drill bit shaft and a straight drive shaft. However, such a coupling, or transmission, has traditionally included several components and required separate lubrication and isolation from the drilling fluid, thus complicating manufacture and maintenance. Therefore, improvements to the drill head of a dual rod drilling system are needed.
To drive the rotation of the drill string, a gearbox having a plurality of motors has traditionally been used. The gearbox can include a gear arrangement that transfers power from the plurality of motors to the inner and out drill rods of the dual rod drilling system. Drilling fluid has also been traditionally introduced at the gearbox to the drill string; however, isolating the drilling fluid from the internal components of the gearbox can be difficult. Further, should a malfunction occur and drilling fluid be introduced to the interior of the gearbox, due to the internal positioning of the gearbox components, it is difficult for an operator to realize this before the components of the gearbox are damaged. Therefore, improvements to the gearbox of a dual rod drilling system are needed.